Electric heating units for warming food or beverage in a container are well known in the prior art. For example, U.S. Pat. No. 3,488,473 discloses a heating device for keeping coffee hot by placing a metal-lined cup on an extension of a base containing a heating element. The heating element comprises a heat-radiating circular plate mounted on the top edge of a hollow cylindrical extension of the base. The coffee cup has a circular recessed opening in its base which receives the circular heating element when the cup is placed on the extension to provide heat transfer contact.
Another heating unit particularly adapted to maintain hot potable liquid at a desired temperature is disclosed in U.S Pat. No. 3,876,861. The heating unit comprises a hollow base of circular cross section made of heat and electrically insulative material. A top wall of the base includes an annular bead, defining a circular opening in which is mounted an inverted pan made of heat conductive material. A thin disc-like plate of positive temperature coefficient resistance material is mounted in heat transfer relationship to the underside of the pan, and a thin sheet of electrically insulative, heat conductive material is interposed between the pan and the plate. Alternatively, the thin sheet of electrically insulative, heat conductive material can be eliminated if the pan is made of electrically insulative material. This patent does not depict the cup intended to be mounted on the heating unit.
The heating devices of U.S. Pat. Nos. 3,488,473 and 3,876,861 both have the disadvantage that no means are envisioned for varying the desired temperature of the beverage to be heated. These references neither disclose nor suggest mechanisms for controlling thermal coupling of heat radiating surfaces in heating elements and receptacle contact surfaces to provide temperature adjustment features. Accordingly, the applications of such devices are limited to providing fixed temperature heating sources.
A complex mechanism for adjusting thermal coupling between an electric plate and the bottom surface of a container is disclosed in U.S. Pat. No. 2,478,614. This patent shows an adjustable cooking plate assembly which comprises a base plate having an upstanding flange in which a series of superimposed plates are telescopically nested. Plates may be added or eliminated as desired to achieve a coarse adjustment of the effective cooking temperature of the uppermost plate. The spacing between the plates is varied by relative rotation of adjacent plates for adjustment of the effective cooking temperature of the uppermost plate. In particular, the means for effecting adjustment of the spacing between plates comprises stepped shoulders formed on the plates which cooperate with depending projections on the undersides of the plates stacked thereon. The projections are adapted for slidable up and down movement on the stepped shoulders in accordance with the relative rotation of the plates.
The adjustable cooking plate assembly of U.S. Pat. No. 2,478,614 has the disadvantage that it is cumbersome and expensive to manufacture due to the multiplicity of plates. The cumbersome and expensive nature of the assembly makes it wholly unsuitable for use in an electric heating device for warming a cup of coffee or a hamburger container, for example.
Accordingly, it is a broad object of the present invention to provide a heating unit having application for heating food and beverage receptacles which is of uncomplex design which overcomes the disadvantages of the aforementioned prior art.
It is a further object of the invention to provide an electric heating unit in which the contents of a container, for example, a coffee mug or a sandwich receptacle, can be heated to variable temperatures through adjustment in relative conformance of receptacle and heating unit contact surfaces.
A more specific object of the invention is to provide a heating unit which includes a planar support surface for beverage and food containers, and a mechanism for effecting thermal coupling of recessed surfaces in the container, for example, concavity in cup base, to effect controlled heat transfer therein.
Yet another object of the invention is to provide food and beverage receptacles which include conductive elements which coact with a heating unit for controlled adjustment of heat transfer therebetween.
Another object of the invention is to provide an appliance for warming the contents of a container which occupies a small area and is suitable for use on desktops and tabletops.